1. Field of the Invention
The invention relates to downhole tools, and specifically relates to a borehole logging tool operable over a range of borehole sizes.
2. Discussion of Prior Art
Well boreholes are typically drilled in earth formations to produce fluids from one or more of the penetrated formations. The fluids include water, and hydrocarbons such as oil and gas. Well boreholes are also drilled in earth formations to dispose waste fluids in selected formations penetrated by the borehole. The boreholes are typically lined with tubular structure commonly referred to as casing. Casing is typically steel, although other metals and composites such as fiberglass can be used. Grouting material, such as cement, fills the casing-borehole annulus to hydraulically isolate various formations penetrated by the borehole and casing.
The wall of the casing can be thinned. Corrosion can occur both inside and outside of the casing. Mechanical wear from pump rods and the like can wear the casing from within. Casing wear can affect the casing's ability to provide mechanical strength for the borehole. In addition or alternatively, various grouting problems can compromise hydraulic isolation of the casing, such as improper bonding, incomplete filling of the casing-cement annulus, and/or casing corrosion/wear.
Measures of one or more of the borehole parameters of interest are useful over the life of the borehole, extending from the time that the borehole is drilled until the time of abandonment. It is therefore economically and operationally desirable to operate equipment for measuring various borehole parameters using a variety of borehole survey or “logging” systems. Such logging systems can include multiconductor logging cable, single conductor logging cable, etc.
Borehole environments are typically harsh in temperature, pressure and ruggosity, and can adversely affect the response of any logging system operating therein. More specifically, measures of the borehole parameters can be adversely affected by harsh borehole conditions. Since changes in borehole temperature and pressure are typically not predictable, continuous and real time system calibration within the borehole is highly desirable. Generally, downhole tools are lowered through the inner diameter of the casing tubing for various purposes. Some tools are provided with power through electrical conductors while other tools are battery-powered. Downhole tools may include a number of modules with lengths up to thirty feet, or even more.
Boreholes are drilled and cased over a wide range of diameters. The casing inside diameter can also vary due to corrosion, wear, or other obstructions. It can be desirable for a borehole tool to operate over a range of borehole diameters.